The present invention relates generally to image forming systems, and specifically relates to charged particle emitting print heads utilized in electron beam imaging printing.
In an image forming system, such as ionography, or electron beam imaging (EBI), a latent electrostatic image is formed on an imaging dielectric surface by directing beams of charged particles onto the surface. The latent electrostatic image thus formed may then be developed by applying toner particles to the imaging surface that are attracted to those areas of the imaging surface where the electrostatic latent image resides. The toner particles on the imaging surface are then transferred to a receiving member (such as paper) before the imaging surface is cleaned in preparation for a new imaging cycle.
The source of the beams of charged particles in the image forming system is a print head. Referring to FIG. 1A, a typical print head 10 includes three layers that have electrodes. A first layer includes a plurality of RF-line electrodes 16 separated from a second layer of finger electrodes 12 by a dielectric layer 14. A third layer is a screen electrode 18 isolated from the finger electrodes by a spacer layer 20. The surface of both the RF-line electrodes 16 and the finger electrodes 12 can be smoothed by a smoothing dielectric 11. In thin film structures, the smoothing dielectric is usually SOG (spin on glass). The finger electrodes 12 have finger openings 13, typically circular, which are generally aligned with the apertures 22 in the screen electrode 18, as shown in FIG. 1A. The RF line electrodes 16 intersect the finger electrodes 12 where the finger openings 13 are located. If a high voltage is applied to the finger electrodes 12 and the RF-line electrodes 16, an electrical breakdown of air inside the finger openings 13 occurs.
Referring to FIG. 1B, a cross-section of a single charge production site of the print head 10 is shown. The electrical breakdown causes formation of gaseous plasma full of charged ions and electrons. While the polarity of particles used for imaging is determined by the polarity of the screen electrode 18 potential with respect to a grounded imaging member 24, on/off switching of charge emission from the print head 10 is regulated by a potential difference between the screen electrode 18 and the finger electrodes 12.
The dielectric layer 14 is typically formed from stoichiometric compounds, such as silicon oxide, silicon nitride, silicon oxy-nitride, aluminum oxide, titanium oxide, boron nitride, etc., or their combination. Electrical conductivity of such materials is very low, about 10xe2x88x9214 S/cm or less at room temperature.
A disadvantage of conventional print heads, and especially print heads designed for high density printing, is that the dielectric layer is subject to degradation. In particular, with repeated printing cycles, the plasma generated in the finger openings 13 degrades the dielectric layer.
Referring to FIG. 2, evidence of the dielectric degradation is shown. Underneath the finger electrode with a circular opening, there can be seen a dielectric layer, which in this particular case is aluminum oxide. The dielectric layer has been subjected to electrical discharges for a time equivalent to printing about 150,000 pages. Significant erosion of the dielectric material can be seen in the amount of dielectric by-products formed in the area around the opening. Such deterioration leads to charge generation reduction and therefore to print quality degradation. Ultimately, such degradation can lead to a full dielectric breakdown of the print head.
For the aforementioned reasons, there exists in the art a need for an electron beam imaging print head less susceptible to degradation arising from plasma generation.
The present invention provides a print head for an image forming system that is resistant to erosion. The print head comprises RF-line and finger electrodes separated by an isolating structure containing a dielectric and a semiconductor or resistive material. For example, the isolating structure may include a dielectric coated with a layer of semiconducting material. Typically, the semiconductor utilized in the present invention has a conductivity between about 10xe2x88x926 and about 10xe2x88x923 S/cm. The semiconductor can be made of a solid solution of a gas in a metal or semiconductor, where the gas includes a hydrogen gas, a nitrogen gas, an oxygen gas, and a halogen gas, or their mixtures. The semiconductor may also include solid solutions of non-metals in a metal, where the nonmetals include boron and/or carbon.